The present invention relates to a spindle clamping device for a machine tool, particularly for clamping a spindle assembly of double-axis type structure, such as one found on a horizontal boring and milling machine.
In a horizontal boring and milling machine of a known structure, various kinds of spindle clamping devices are utilized, each having an outer milling spindle rotatably supported and an inner spindle also rotatably supported inside the milling spindle so as to reciprocate along the axial direction of the milling spindle. For instance, one example of such a horizontal boring and milling machine is provided with a first shaft as an outer milling spindle and a second shaft as an inner boring spindle. And in such example, the spindle clamping methods are classified into three manners, that is, (1) a method in which a .pressurized oil is always supplied to a static pressure pocket portion disposed between the milling spindle and the boring spindle for clamping the first shaft, (2) a method in which an oil ring is disposed between the first and second shafts and a pressurized oil is poured to the oil ring thereby clamping the first and second shafts, the oil ring being of a structure such as shown in the Japanese Utility Model Publication No. 62-43684, and (3) a method in which the first and second shafts are clamped by incorporating a self-pressurized oil retaining mechanism into the second shaft, such as disclosed in the Japanese Utility Model Publication (JITSUYO KOKOKU) No. 62-43684.
However, in the above prior art methods of (1) and (2), in which the pressurized oil is always supplied to the static pressure pocket portion disposed between the first shaft rotatably supported by a spindle head and the second shaft is disposed rotatably inside the first shaft to be reciprocal along the axial direction for clamping the first shaft, the pressurized oil must be always supplied or always retained during the rotations of the spindles. For this reason, a large quantity of heat is generated for the static pressure pocket portion during a cutting operation of the machine tool by rotating the first shaft with high speed. Accordingly, the first shaft is expanded in its axial direction by the thermal expansion of the static pressure pocket portion, which adversely affects the working precision of the machine tool.
Furthermore, in the above prior art method of (3), in which the pressurized oil is supplied to the oil ring disposed between the first shaft rotatably supported by the spindle head and the second shaft disposed rotatably inside the first shaft to be reciprocal along the axial direction for clamping the first and second shafts, not avoided to prevent the pressurized oil from leaking during the rotation of the shafts cannot be avoided, thus this method is not suitable for long term operation of the machine tool. In order to obviate these defects, in the prior art, a mechanism for supplying the oil to the oil ring must be provided by positioning the second shaft to a predetermined angular position and making coincident a phase of the oil pouring unit with a phase of an oil supplying unit. Therefore, when the pressurized oil leaks, the pressure to the oil ring is lowered, which makes it difficult to surely clamp the spindle with high rigidity. Furthermore, the oil supply to the oil ring requires much additional time, resulting in the lowering of the workability.
Other various attempts have been made in the prior art to provide an improved spindle clamping device for machine tools provided with a milling spindle and boring spindle particularly for the reason that insufficient rigidity of the boring spindle per se directly affects on cutting efficiency of the machine.